The Collaborative International Dictionary of English v.0.48:

Monotonous \Mo*not"o*nous\, a. [Gr. ?; mo`nos alone, single + ?
   tone. See Tone.]
   Uttered in one unvarying tone; continued with dull
   uniformity; characterized by monotony; without change or
   variety; wearisome. -- Mo*not"o*nous*ly, adv. --
   Mo*not"o*nous*ness, n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

monotonous
    adj 1: tediously repetitious or lacking in variety; "a humdrum
           existence; all work and no play"; "nothing is so
           monotonous as the sea" [syn: humdrum, monotonous]
    2: sounded or spoken in a tone unvarying in pitch; "the owl's
       faint monotonous hooting" [syn: flat, monotone,
       monotonic, monotonous]

Moby Thesaurus II by Grady Ward, 1.0:

113 Moby Thesaurus words for "monotonous":
   alliterating, alliterative, articulated, assonant, banal, banausic,
   belabored, blah, boring, broken-record, catenated, ceaseless,
   chanting, chiming, cliche-ridden, colorless, commonplace,
   concatenated, connected, constant, continual, continued,
   continuing, continuous, cyclical, dim, dingdong, direct, drab,
   drearisome, dreary, dry, dryasdust, dull, dusty, endless,
   everlasting, everyday, featureless, gapless, gray, harping, ho-hum,
   humdrum, immediate, incessant, interminable, invariable,
   jingle-jangle, jog-trot, jogtrot, joined, jointless, labored,
   linked, long-winded, mechanical, monotone, monotonic, never-ending,
   nonstop, ordinary, pedestrian, perennial, periodic, poky, prolix,
   prosaic, recurrent, repetitious, repetitive, rhymed, rhyming,
   round-the-clock, routine, run-of-the-mill, running, same, samely,
   seamless, serried, singsong, sleep-inducing, smooth, soporific,
   stable, steady, stodgy, straight, tedious, tiresome, tiring,
   treadmill, twenty-four-hour, unbroken, unceasing, undifferentiated,
   unending, uneventful, unexciting, uniform, uninteresting,
   unintermitted, unintermittent, unintermitting, uninterrupted,
   unrelieved, unremitting, unstopped, unvaried, unvarying, wearisome,
   wearying